Cretaceous, island-arc type, magmatic evolution resulted in the formation of the Srednogorie volcanic intrusive zone. The Chelopech mineral deposit is located within the Panagyurishte metallogenic district, a central part of the Srednogorie zone.

The Chelopech region consists of a Precambrian metamorphic basement consisting of gneisses, amphibolites, and metasediments overlain by Upper Cretaceous, volcanosedimentary sequences which include the Chelopech formation; the primary host to mineralisation.

The Chelopech Formation reaches thicknesses of up to 2,000 m and consists of Lower and Upper units.

The Lower Chelopech Unit is comprised of a basal sequence of siltstones and calcareous argillites with subordinate terrigenous sandstones and angular conglomerates. Upwards these sediments become intercalated and eventually superseded by volcanic sequences including andesites, andesitic agglomerates, andesitic lapilli and psammitic tuffs.

The Upper Chelopech Unit of Coniacian-Santonian age (Lower Senonian) comprises a complex of andesitic and dacitic lavas and tuffs with siliciclastic, volcaniclastic and argillaceous sediments intruded by sub-volcanic bodies of porphyritic andesites. The Upper Chelopech Formation passes up and laterally from mixed (terrigenous-volcanogenous) gritty sandstones, with volcanogenic exhalative iron-manganese oxide horizons, into volcanogenic talus breccias and agglomeratic tuffs of andesitic affinity.

Three successive mineralisation stages have been recognised at Chelopech, including an early Fe-S stage consisting mainly of disseminated and massive pyrite, a second Cu As-S stage which is the economic Cu and Au stage, and a late Pb-Zn stage. These display different geometries, including veins, breccias, massive and disseminated sulphides.

The mineralisation occurs in a range of different morphologies, including lens-like, pipe-like and columnar bodies that typically dip steeply towards the south. The mineralised zones vary from 40 to 200 m in length, are 20 to 130m thick, and can extend at least 390 m down plunge.

Production from underground is attained via Sublevel Longhole Open Stoping. The various ore bodies are developed at nominal 30 m vertical intervals and accessed by major declines in both, the Western and Central areas. Stopes are designed to be 20 m wide between the levels. The length of the stope depends on the geotechnical conditions, but can range between 20 and 60 m. The new trend of stope design is to keep a 20-30m length and 60m height, where geological and geotechnical conditions are suitable. This allows for improvement in ore handling and dust suppression during ore mucking as a result of shorter remote loading.

Once mined via an “end-slice” methodology, stopes are backfilled with “paste fill” produced from the mill tailings, to which cement is added and which is gravity fed underground via a system of borehole and pipes to the stopes being filled.

Operations upgrades commenced in 2009, with the installation of the first hydraulic mine back-fill plant, subsequently upgraded to the current paste plant in 2010. Process plant upgrades continued through 2010 in preparation for the increased tonnages from the upgrade of the mine. A new grinding circuit replaced the original secondary and tertiary crushing circuits, together with the installation of a new rougher/scavenger flotation bank. The existing flotation cells were converted into an expanded 3 stage cleaner circuit, with the upgraded circuit commencing operations in February 2011. Flotation tailings continue either being dewatered to produce “paste” for backfilling the mined out stopes underground, or deposited in the upgraded TMF facility as required.

Further upgrades were completed in 2012 with replacement second and third cleaner stages in the copper circuit, and a complete new pyrite recovery circuit, and concentrate conveying and rail loadout handling system both completed in 2014.

Crushed product from the primary crushers, which has a typical P80 of 100 mm, is ground using a single-stage closed grinding circuit with cyclone classification. This comprises a single stage SAG mill, 8.53 m diameter x 4.72 m long, with a rated capacity of 5,800 kW. Cyclone underflow is returned to the SAG mill and the overflow gravitates to the flotation circuit passing via an “in-stream” analysing system, which monitors the density and the assay composition of the stream, and a particle size analyser.

The flotation process continues as before in the new rougher/scavenger circuit comprising of four 100 m3 tank cells, where a bulk sulphide concentrate, containing the copper minerals and the majority of pyrite, is collected and forwarded to the cleaner circuit.

The combined concentrate flows via a conditioner tank to the previous rougher/scavenger cells, rearranged to form the new first cleaner circuit, by using lime for pyrite depression. These comprise of two banks of four, 4-cell Denver-500 cells, and the circuit tails (Cleaner tails) being combined with the rougher/scavenger tailings to form the final tailings stream. The first cleaner concentrate reports to the second and third cleaners, while the cleaner tailing reports back to the first cleaner feed.

Reagents currently used are PAX (potassium amyl xanthate) for collection, Oreprep F549 for frother, and slaked lime for pH control. Final concentrate is gravity fed to the dewatering section, while the final tailings are transported by gravity to the current water recovery thickener located at the plant site.

The copper concentrates report to the filter section for thickening and filtration. A 12 m high-rate thickener is used to thicken the final copper concentrate, which is then dewatered typically to a moisture content of less than 8%, using a vertical plate pressure filter. The filtered cake is stored and transported periodically by rail to Burgas, for onward shipment to the Smelter located in Namibia.

Concentrator Upgrades – post 2012

Cleaner Circuit (completed July 2012)
In mid-2013, the existing second and third copper circuit cleaner banks cells were replaced with new units. Each stage comprises of 2 stages of ‘staged flotation reactors’ (SFRs) in series for each. Selection of these units followed the extensive plant trials through 2012 testing a production sized unit as the first stage of the second cleaner (Woodgrove Technologies, 2012).

Pyrite Recovery Circuit (completed March 2014)
Prompted by the success of the cleaner circuit upgrade, the new pyrite circuit included the SFR design as the flotation units. The remainder of the circuit includes a concentrate thickener, filter, and concentrate storage area located on the west side of the current concentrator building. The complete circuit was commissioned and in full production by the end of Q2 2014.

Concentrate Handling Facility (completed Q3, 2014)
This material handling system conveys both of the copper and pyrite concentrates produced from their respective storage areas, across the site to a ‘rail loadout’ system. From here, the two concentrates are transported to a holding warehouse in the port of Burgas, from where it is loaded into bulk cargo carriers for transport to the final destination.